This invention relates to the treatment of water, and more particularly to the production of potable water by reverse osmosis after hard water has been softened by ion exchange. The softening of hard water by ion exchange results from the replacing of magnesium and calcium ions in the hard water by sodium ions from an ion exchange material. Water which has over about three grains of hardness is not considered soft and water having a total dissolved solids content over about 500 parts per million is undesirable for potable use.
Although hard water can be completely softened by ion exchange, the amount of dissolved solids in the soft water is essentially the same as that in the hard water. Thus, when the total dissolved solids in the hard water exceeds the 500 p.p.m. limit considered desirable for potable use, mere softening by ion exchange will not render the water potable. Treatment of water by reverse osmosis ordinarily removes about ninety percent of the total dissolved solids in the water, and thus is capable of producing potable water. However, treatment by reverse osmosis results in a stream of waste water having a higher concentration of dissolved solids than the water being treated. The waste stream is ordinarily discarded as useless, or it may be employed for a low importance use such as watering lawns. Also, the need for a reverse osmosis system to operate continuously for long periods has hindered integration with other types of water treatment.